This invention and embodiments thereof relate to a radiation emitting device and system and in particular an X-ray tube. More particularly, the invention and embodiments thereof related to a tube for a mammography apparatus which tube and apparatus may comprise an improved filtering system.
In radiology, particularly human radiology and particularly in mammography, an X-ray tube is used to irradiate a body and reveal an image of the irradiation on the side of the body that is opposite to the side on which the tube is placed. An X-ray tube comprises a cathode emitting electrons and projecting them at high speed onto an anode as a result of the very high voltage between the cathode and the anode. The anode, subjected to electron bombardment, emits X-rays, in general almost omni-directionally. An X-ray tube is normally shielded except at the position of an emission window through which the useful X-rays are emitted. Due to emission characteristics, variations in the high voltage and the more or less pure nature of the anode material, the frequency spectrum of the X-rays emitted is not a single value but a continuum around a central value.
Developments in radiology have led to attempts to confine this emission spectrum in a narrow band. Depending on hardness and frequency, the X-rays emitted are absorbed in the body by materials of different natures. To characterize a material, especially a tissue in the human body, it is then necessary to use the narrowest possible emission value around an expected value. Since the emitted spectrum is broad, this result is obtained by interposing filters in the path of the X-rays, before they reach the body to be studied. The filters which may be made of aluminum, copper, molybdenum, tungsten, beryllium or rhodium, or even an alloy of these substances, blocks certain parts of the radiation and lets through useful radiation that it is more appropriate to the examination to be made.
There is a known way of positioning a roundabout facing the emission window of the tube. This roundabout is typically a wheel with blades, each blade comprising a filter that, for a given rotational position of the wheel, takes a position before the emission window to bring about the expected filtering. The use of such a wheel is practical because it is not really possible to envisage the manual replacing of the filters at the emission window of the X-ray tube. Since the X-ray tube is subjected to very high voltage, it is necessary to take either preliminary precautions for disconnecting the tube (entailing a loss of time) or additional precautions of electrical installation (increasing the complexity of manufacture of the tube).
However, such a roundabout or filter wheel is not entirely satisfactory. First of all, especially in the context of mammography, this roundabout is a bulky accessory. In the context of a mammography, in practice, the patient places her head beside the x-ray tube side so that her breast can be in the path of the X-rays produced. Experience shows that the patient has to make contortions owing to the fact that the X-ray tube naturally takes up much space. The roundabout takes up even more space and its use is therefore undesirable.
Furthermore, in the field of mammography especially, such a structure is no longer satisfactory for another reason: for a given type of examination, a specific filter may be required. However, the absorption capacity of the filter has to be modified, firstly according to the size of the breast and, secondly, according to the nature of the examination made. In practice, placing a filter of variable thickness in the path of the X-rays does this. Taking five to ten possible thicknesses for each filter soon results in an excessively huge battery of available filters, given that each of these filters, owing to its quality, is particularly costly.